A thermal transfer printer performs printing by using a thermal head to heat a predetermined portion of an ink ribbon coated with a molten ink or a sublimation dye, and thermally transferring the ink to printing paper. Because this thermal head transfers the ink by generating heat, heat accumulates in the head over the course of printing. The temperature of the thermal head is determined by the balance between this heat accumulation and heat radiation. Therefore, if more heat accumulates than is radiated, the temperature of the thermal head will steadily rise. When the temperature of the thermal head rises, then even if a print command is issued for a given print density, the density at which the thermal head actually prints will end up being higher than the print density when the thermal head is cooler.
In view of this, in Patent Document 1 is disclosed a method for powering a thermal head in which the optimal printing energy is calculated while the temperature of any heat generating member is measured.
Meanwhile, when the temperature around the printer is low, the overall print density will be lower, so there is also a known printer that detects the ambient temperature and adjusts the print density.
Also, with a thermal printer that makes use of an ink ribbon, a problem has been indicated whereby the ink ribbon shrinks in some places due to heating of the thermal head, and this creates wrinkles in the ink ribbon and diminishes print quality (see Patent Document 2).
This Patent Document 2 cites as prior art that with a label printer, which uses a thermal head to print on rolled label paper in which labels are affixed at regular intervals to a base paper, to prevent the ink ribbon from becoming wrinkled, the conveyance speed of the ink ribbon is matched to the conveyance speed of the paper, the winding diameter of the winder shaft, and the winding diameter of the ink ribbon feed shaft, and a suitable torque is applied to the motor that drives the feed shaft and winder shaft according to this conveyance speed, thereby achieving the proper tension of the ink ribbon.
Patent Document 2 also indicates that since more of the ink ribbon will shrink with a printed image having a large printed surface area that has to be heated, wrinkling happens at the boundary between the portions heated by the thermal head and portions not heated, and it is difficult to eliminate wrinkles merely by adjusting the ink ribbon to the proper tension by optimizing the torque applied to the motor that drives the feed shaft and winder shaft. It is also stated that when the original image has a continuous pattern, in a homogeneous pattern is continuously laid out in the lateral width direction, if the original image of this continuous pattern is written to a drawing memory so as to be printed past the ends of the print medium, the boundary between the heated portions and unheated portions will be away from the print medium, and the places on the thermal transfer ink member where wrinkling occurs will be away from the print medium, thereby eliminating the effect of the wrinkles.
Patent Document 1: Japanese Laid-Open Patent Application H4-358853
Patent Document 2: Japanese Laid-Open Patent Application 2002-254687
In Patent Document 2 mentioned above, the boundary between the heated and unheated portions is moved away from the print medium, which moves the places where wrinkles form on the thermal transfer ink member away from the print medium, and this is supposed to keep wrinkles from forming and eliminate the effect of wrinkles, but a continuous pattern in which a homogeneous pattern is continuously laid out in the lateral width direction needs to be present in the original image, so a problem is that this approach cannot be applied if no such continuous pattern is present in the original image.
Also, reducing the energy supplied to the thermal head is expected to suppress wrinkling of the ink ribbon caused by the heating of the thermal head. However, when the ambient temperature is low, in order to keep the overall print density from being low, print density adjustment is performed in which the print density is controlled in the direction of being raised, and this adjustment is in the opposite control direction from that of control in which the wrinkling of the ink ribbon is suppressed, so a problem is that this approach cannot be applied when the ambient temperature is low.